Copy sheet guide device in electrophotographic copying machine

ABSTRACT

In an electrophotographic copying machine of the image transfer type, a guide device for guiding a transfer medium (imagereceiving copy sheet) toward a photosensitive medium or drum from which the image is transferred, comprising transport roll means and a pair of parallel upper and lower transfer medium guide plates provided between the transport roll means and the photosensitive medium and adapted to guide a transfer medium between the guide plates. The upper guide plate, which lies adjacent to the back side of the transfer medium in the portion of its path immediately before image transfer, has the forward end edge thereof formed into a convex curve with respect to the surface of the photosensitive medium. The forward end edge of the lower plate may be straight or concave.

United States Patent [191 Nitanda et a1.

[ COPY SHEET GUIDE DEVICE IN ELECTROPHOTOGRAPHIC COPYING MACHINE [75] Inventors: Hiroshi Nitanda; Hiroyuka Hattori;

Koichi Miyamoto, all of Tokyo, Japan [73] Assignee: Canon Kabushiki Kaisha, Tokyo,

' Japan 22 Filed: Nov. 21,1972 211 App]. N0.Z 303,571

[30] Foreign Application Priority Data 3,516,657 6/1970 Knudsen .11.. 271/68 June 28, 1974 3,620,615 11/1971 Volkers 355/3 Primary Examiner-Samuel S. Matthews Assistant Examinerl(enneth C. Hutchison Attorney, Agent, or Firm-Fitzpatrick, Cella, Harper & Scinto 5 7 ABSTRACT In an electrophotographic copying machine of the image transfer type, a guide device for guiding a transfer medium (image-receiving copy sheet) toward a photosensitive medium or drum from which the image is transferred, comprising transport roll means and a pair of parallel upper and lower transfer medium guide plates provided between the transport r011 means and the photosensitive medium and adapted to guide a transfer medium between the guide plates. The upper guide plate, which lies adjacent to the back side of the transfer medium in the portion of its path immediately before image transfer, has the forward end edge thereof formed into a convex curve with respect to the surface of the photosensitive medium. The forward end edge of the lower plate may be straight or concave.

4 Claims, 10 Drawing Figures PAIENIEnJum m4 SHEET 1 (IF 3 PATENTEnJuuza m4 SHEET 2 OF 3 FIG. 3

FIG. 4

PATENTEflJum m4 SHEEI 3 ()F 3 FIG.5'

FIG. .7 (LEADING EDGE) (TRAILING EDGE) FIG. 6

FIG. 8

(LEADING EDGE) RAILING EDGE) FIG. 9

(LEADING EDGE) 0' Q' NG EDGE) (TRAIL) COPY SHEET GUIDE DEVICE IN ELECTROPHOTOGRAPHIC COPYING MACHINE BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a guide device for guiding an image transfer medium toward the surface of a photosensitive medium in an electrophotographic machine, and more particularly to a guide device adapted to guide the transfer medium for uniform intimate contact with the surface of the photosensitive medium.

2. Description of the Prior Art With an electrophotographic copying machine of the image transfer type, it has often been the case that blank or unimaged areas are created on the transfer medium due to the non-flatness thereof. This detracts from the unique feature of the image transfer type copying machine which is characterized by its ability to freely transfer images onto any type of transfer medium. To avoid this, heretofore, a special type of copy paper having a good degree of surface flatness which is stable for both higher and lower temperatures has been prepared for exclusive use with such type of copying machine. Also, in such a copying machine, the mechanism for supplying transfer mediums to the surface of the photosensitive medium is required to operate with a high degree of accuracy, thus making the entire machine economically disadvantageous.

SUMMARY OF THE INVENTION It is an object of the present invention to overcome the above-noted disadvantages which have existed in the prior art.

It is another object of the present invention to provide a transfer medium guide device in an electrophotographic copying machine of the image transfer type using a drum type photosensitive medium which is improved to guide a transfer medium for uniform intimate contact with the surface of the photosensitive medium to provide a good copy without any blank or unimaged area left thereon even if the transfer medium is of poor surface flatness, wherein the device is simple in construction without requiring any special design consideration to be 'given to the accuracy of the transfer medium conveyor system.

According to the present invention, the transfer medium guide device for an electrophotographic copying machine of the image transfer type comprises a pair of upper and lower guide plates provided between transfer medium transport roll means and a photosensitive medium and adapted to guide a transfer medium through a clearance between the two guide plates toward the surface of the photosensitive medium. The upper guide plate (for guiding the back side of the transfer medium) has the forward end edge thereof formed into a convex curve with respect to the surface of the photosensitive medium, and the lower guide plate (for guiding the image-receiving surface of the transfer medium) has the forward end edge thereof formed into a straight line or a concave curve with respect to the surface of the photosensitive medium. The result is that blank or unimaged areas which would otherwise occur in a copy may be completely eliminated or substantially minimized even if the transfer medium in use is of poor surface flatness. Furthermore, the

present invention eliminates the need for special considerations to be given to the transfer medium conveyor system in terms of the accuracy thereof and thus can achieve the intended purposes satisfactorily.

Furthermore, when a liquid developing system is employed, the surface of the photosensitive medium is normally wet with the developing liquid, which tends to cause the transfer medium to become wrinkled during its contact with the surface of the photosensitive medium. These wrinkles, once formed, are difficult to smooth and may thus result in unimaged or blank areas in the resultant copy. This problem is also overcome by the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will become fully apparent from the following detailed description thereof taken in conjunction with the accompanying drawings, in which:

FIG. l-is a schematic view illustrating the construction of an electrophotographic copying machine of the image transfer type using a drum'type photosensitive medium to which the present invention is applicable;

FIGS. 2a and 2b illustrate the transfer medium guide device according to an embodiment of the present invention;

FIGS. 3 to 6 are views all taken along line AA in FIG. 2a and respectively show different forms of the forward end edges of the guide plates. FIGS. 3 to 4 illustrate undersirable forms of such guide plates while FIGS. 5 to 6 illustrate preferred forms of such guide plates according to the present invention;

FIGS. 7 and 8 show the manner in which unimaged or blank areas are created in a copy provided when the devices of FIGS. 3 to 4 are employed; and

FIG. 9 shows the manner in which minor unimaged areas may sometimes be created when the device of FIG. 5 is employed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS lamp 24 and the image of the original is directed via a mirror 25 and a mirror lens 26 toan exposure station 27, where the image is focused on a photosensitive drum 17. The photosensitive medium forming the surface of the drum 17 comprises a transparent dielectric layer overlaid on a photosensitive layer. The photosensitive drum I7 is rotatable in the direction indicated by an arrow and may be positively charged by a primary charger 29 which is supplied with a high voltage from a high voltage source 28. Then-the image of the original formed'on the drum 17 may be subjected to a slitexposure at the exposure station 27, and simultaneously therewith, it may be AC-discharged by a discharger 30 supplied with a high AC voltage from the voltage source 28. Thereafter, the formed image on the photosensitive medium may be subjected to an overall exposure by a lamp 31, thus providing an electrostatic latent image. These processes may be replaced by a series of processes such as primary DC charging, simultaneous application of image light and secondary charge of the opposite polarity, and overall exposure, in the named order. Alternatively, the secondary charging process may immediately follow the application of image light.

The electrostatic latent image may then be developed by a body of developing liquid 33 in a developing device 32, whereafter the photosensitive medium is corona-charged by a high positive voltage from a postcharger 38 without the image thereon being disturbed so as to squeeze out the excess developing liquid. A transfer medium P may be fed from a transfer medium supply station by a roll 46 or the like, and brought into intimate contact with the surface of the photosensitive drum 17, whereupon positive corona charge is applied thereto from a charger 39 to transfer the image from the photosensitive drum to the transfer medium. Thereafter, the transfer medium P may be separated from the photosensitive drum by means of a separator belt 40 and then delivered to a drying-fixing station 41 by a belt 57. Any residual toner or developing liquid on the photosensitive drum 17 may be wiped off by the edge portion 42 of blade cleaner 42. The described processes may be repeated for a subsequent copy cycle.

The transfer medium P may then be heated just beneath a heater 58 while being dried and fixed by warm air blown to the vicinity of the heater from a duct 54 through its outlet 55 Thereafter, the transfer medium P may be electrostatically discharged by a discharger 60 to remove any residual electrostatic charge on the surface thereof, and then directed by an outlet roll 61 so as to pass'through an outlet 62 and discharge along an auxiliary tray 8 onto the lid 7 of transfer medium cassette 6 which also serves as a tray.

In order to convey a transfer medium P toward the photosensitive drum 17 in the above-described copying machine, the present invention provides a pair of lower and upper guide plates 201 and 202 between the transfer medium transport roll 46 and the photosensitive drum 17 so that a transfer medium P leaving a set of transport rolls 47 and 47 may pass through a clearance between the guide plates 201 and 202 toward the photosensitive drum 17.

The relationship between the transfer discharging ends of the guide plates and the drum 17, as shown in an enlarged view in FIG. 2A, satisfies certain dimensional requirements as described hereinafter. Similarly, those same dimensional requirements apply to the modification of the preferred embodiment illustrated in which are adjacent to the image bearing photosensitive drum are both formed rectilinearly as shown in FIG. 3, it has been found that the copy image on the transfer or copy medium P may leave thereon a large blank or unimaged area extending from the intermediate portion to the trailingedge thereof, as shown by a shaded portion a in FIG. 7, even if the transfer medium conveyor system is assembled with a considerably high accuracy. This is believed to result from the reasons set forth below. A bending stress may be present in the transfer medium itself when the leading edge thereof shifts from the forward ends of the guide plates to the surface of the drum 17, to thereby prevent the leading edge of the transfer medium from coinciding with the generating line of the cylindrical surface of the drum (i.e., the leading edge of the transfer medium describes a curve when initially contacting the drum surface), or the surface nonflatness of the transfer medium may prevent it from making a good contact with the drum surface; such factors which would cause wrinkling of the transfer medium tend to stay in the leading edge portion of the transfer medium which is fixed (i.e., electrostatically brought into firm contact with the drum 17 by the transfer charger 39) (FIG. 1), thus wrinkling up the trailing edge portion of the transfer medium which is free. On the other hand, no such blank or unimaged area occurs in a thicker and accord ingly more self-supporting transfer medium of better surface flatness (the greater self-supporting property of such thicker medium is useful to prevent the aforesaid wrinkle-making factors from being retained by the medium).

If, as shown in FIG. 4, the transfer-medium discharging end edges of the lower and upper guide plates 201 and 202 are made slightly convex with respect to the drum surface to provide a clearance progressively increasing toward the opposite side edges, the aforesaid wrinkle-making factors will be permitted to escape transversely of the transfer medium before they are permanently retained due to the action of the transfer charger, and at the same time the center portion of the transfer medium will be allowed to bear forcibly against the drum. Accordingly, the blank or unimaged area a as shown in FIG. 7 may be reduced when the guide plates of FIG. 4 are used. It has been found that a greater curvature for the convex end edges of the upper and lower guides 201 and 202 further reduces the unimaged or blank area a, but a stress is then concentrated more intensely in the leading edge of the transfer medium to create an additional unimaged area b therein as shown in FIG. 8.

If, as shown in FIG. 5, the forward end edge of the guide plate 202 for guiding the back side of the transfer medium is made into a convex contour similar to that shown in FIG. 4 while the forward end edge of the other guide plate 201 for guiding the image-receiving surface of the transfer medium is made into a straight line, then the stress imparted to the transfer medium may be reduced to thereby provide a better result in minimizing the creation of the unimaged area. In this case, however, some small unimaged areas might sometimes occur in the trailing edge portion of the transfer medium, as shown at a in FIG. 9, due to the surface non-flatness of the transfer medium at the trailing edge thereof.

When, as shown in FIG. 6, the forward end edge of the guide plate 202 for guiding the back side of the transfer medium is formed into a convex contour while that of the guide plate 201 for guiding the imagereceiving surface of the transfer medium is formed into a concave contour, then the stress imparted to the transfer medium at the leading edge thereof will be reduced further than in the case of FIG. 5. In addition, when the trailing edge of the transfer medium passes through the clearance between the guide plates, first the center portion and then the opposite side portions, of the trailing edge, will leave the guide plates and come into contact with the drum 17. It has been found that this can eliminate even such small unimaged areas a as shown in FIG. 9.

The present invention is based on the abovedescribed findings. Thus, the forward end edge of the guide plate 202 for guiding the back side of the transfer medium P is formed into a convex curve with respect to the drum surface while the forward end edge of the guide plate 201 for guiding the image-receiving surface of the transfer medium is formed into a straight line parallel to the drum surface or a concave curve with respect to the drum surface.

As a result, the unimaged area or areas which would otherwise occur in a final copy may be completely eliminated or substantially minimized even if the transfer medium in use is of poor surface flatness. Furthermore, the present invention eliminates the need for special design considerations to be given to the accuracy of the transfer medium conveyor system and can thus achieve the intended purposes very well.

In FIGS. 5 and 6, the practicable range of curvature for the convex or concave end edges of the guide plates 201, 202 is variable in dependence on such factors as the initial angle of contact of the transfer medium with the drum surface, the clearance between the guide plates 201 and 202, and the distance from the transport rolls 47 47 (FIG. 1) to the initial point of contact between the transfer medium and the drum surface. Experiments carried out for a wide variety of transfer mediums have shown that the following relationships are preferable l 0 mm,

l 0.5 8mm,

1 l 6 mm, where I is the distance between the center of the forward end edge of the guide plate for guiding the imagereceiving surface of the transfer medium and the surface of the photosensitive drum, 1 is related to the curvature of the same end edge and is defined as the difference in length between the centerportion and one of the side edges of that place, l is the distance between the center of the forward end edge of the-guide plate for guiding the back side of the transfer medium and the surface of the drum, and l is the curvature of the forward end edge of the latter guide plate and is defined as the difference between the center portion and one of the side edges of that plate.

We claim: 1. In a copying machine of the image transfer type, said machine having an image bearing medium, a copy medium guide device for guiding a copy medium toward said image bearing medium, said device comprising:

copy medium transport roll means; and a pair of copy medium guide plates disposed in parallel with a spacing therebetween and provided between said transport roll means and said image bearing medium to receive a copy medium there between and to guide said copy medium into contact with said image bearing medium, wherein each said plate has a copy-medium-receiving end edge adjacent said roll means, and a copy-medium-- discharging end edge adjacent said image bearing medium, and wherein one of said guide plates is for guiding the back side of the copy medium and has its said discharging end edge formed into a convex curve with respect to the surface of said image bearing medium.

. 2. A copy medium guide device as set-forth in claim 1, wherein the other one of said guide plates is for guiding the image-receiving surface of the copy medium and has its said discharging end edge thereof formed into a straight line parallel to the surface of said image bearing medium.

3. A copy medium guide device according to claim 1, wherein the other oneof said guide plates is for guidingthe image-receiving surface of the copy medium and has its said discharging end edge'thereof formed into a concave curve with respect to the surface of said image bearing medium.

4. A copy medium guide device according to claim. 

1. In a copying machine of the image transfer type, said machine having an image bearing medium, a copy medium guide device for guiding a copy medium toward said image bearing medium, said device comprising: copy medium transport roll means; and a pair of copy medium guide plates disposed in parallel with a spacing therebetween and provided between said transport roll means and said image bearing medium to receive a copy medium therebetween and to guide said copy medium into contact with said image bearing medium, wherein each said plate has a copymedium-receiving end edge adjacent said roll means, and a copymedium-discharging end edge adjacent said image bearing medium, and wherein one of said guide plates is for guiding the back side of the copy medium and has its said discharging end edge formed into a convex curve with respect to the surface of said image bearing medium.
 2. A copy medium guide device as set forth in claim 1, wherein the other one of said guide plates is for guiding the image-receiving surface of the copy medium and has its said discharging end edge thereof formed into a straight line parallel to the surface of said image bearing medium.
 3. A copy medium guide device according to claim 1, wherein the other one of said guide plates is for guiding the image-receiving surface of the copy medium and has its said discharging end edge thereof formed into a concave curve with respect to the surface of said image bearing medium.
 4. A copy medium guide device according to claim 1, wherein said image bearing medium comprises a photosensitive drum. 